1. Field of the Invention
The present invention relates to inorganic porous membranes adapted for use in filtration, gas separation or the like and a process for the production of the inorganic porous membranes.
2. Description of the Prior Art
As one of such inorganic porous membranes as described above, there has been proposed a multilayer inorganic porous membrane consisting of a monolayer or multilayer porous support integrally provided on at least one surface thereof with a porous thin layer having an average pore size smaller than that of the porous support. In general, this kind of inorganic porous membranes are required to be superior in corrosion resistance and heat resistance. In the case that high precision is required for filtration and separation, it is important to provide the inorganic porous membrane without causing any pin holes and cracks in its porous thin layer. In the case that the inorganic porous membrane is exposed to high temperatures, acid, alkali or the like in use for filtration and separation or in its regenerative process, it is required to be superior particularly in heat resistance. In a particular process for the production of superpure water in a semiconductor field or the production of pyrogen free water in a pharmaceutical field, however, a problem will be caused due to dissolution of components from the inorganic porous membrane.
In a regenerative process of the inorganic porous membrane for eliminating contaminants adhered thereto in a field of ultrafiltration or microfiltration, high resistance to corrosion is required for washing the porous membrane with acid and alkali. In a field where the inorganic porous membrane is used for ultrafiltration or microfiltration of fine particles such as organic colloid, the inorganic porous membrane, for instance, is washed with acid and alkali and with steam for sterilization. If pin holes, cracks or the like exsist in the thin layer of the porous membrane, they will increase during washing, resulting in deterioration of precision of the filtration and the resistant property to corrosion.
Such an inorganic porous membrane as described above is known from the following publications. In the Japanese Non-examined Patent Publication No. 60-156510 (U.S. Pat. No. 4,711,719) dislcoses a process for the production of an inorganic semi-permeable membrane by coating a porous support of sintered inorganic oxides with a suspension of coating material forming an inorganic thin layer and then heating the intermediate product. In this production process, the suitability of the porous support for forming the thin layer is determined mainly by an average pore size of the support. If the average pore size is too large, sol particles in the suspension permeate into the porous support and do not form any thin layer on the support. For this reason, it is pointed out in the publication that a preferable pore size of the support is about from 0.1 to 0.5 micron. For the purpose of preventing the forming of cracks during the production process, the intermediate product is dried by a complicated supercritical drying for a long time and is gradually heated during the firing process thereof so that a porous thin layer of .gamma.-alumina is formed on the porous support to obtain an ultrafiltration membrane.
In the Japanese Non-examined Patent Publication No. 52-94572 there is disclosed a multilayer inorganic porous membrane consisting of a porous support coated with a porous thin layer, the porous support having an average pore size of from 10 to 200,000 times, preferably from 200 to 20,000 times that of the thin layer. A heat resistant inorganic porous membrane for gas separation is disclosed in the Japanese Examined Patent Publication No. 61-27091, and a filter in the form of a heat resistant inorganic porous membrane for liquid and gas filtration is disclosed in the Japanese Non-examined Patent Publication No. 61-500221. The former inorganic porous membrane consists of a porous support of heat resistant oxide having a pore size of from 1500 to 5000 angstrom integrally formed thereon with a sintered layer having an average pore size of from 200 to 1200 angstrom, the sintered layer containing aluminum oxide particles amounts of 97% by weight and having particle sizes of less than 0.5 micron. However, there is not disclosed any definition concerning the composition of the porous support and sintered layer. With regard to the composition of the support, it is noted that an example of the support contains mullite of 8% matrix and aluminum oxide with an impurity of 0.5% by weight. In the support, however, a big problem will be caused in corrosion resistance. With regard to the composition of the sintered layer, it is noted that an impurity in the layer is 3% by weight in maximum. However, the sintered layer as well as the support has a big problem in corrosion resistance and in dissolution of the impurity. With regard to the aluminum oxide forming the sintered layer, it is noted that .alpha.-alumina of less than 0.5 micron is preferably used for the layer. In general, however, .alpha.-alumina having a particle size of more than 0.2 micron has a specific surface area less than 10m.sup.2 /g, and .gamma.-alumina having a particle size of less than 0.2 micron has a specific surface area more than 10m.sup.2 /g. Accordingly, .gamma.-alumina having a particle size of less than 0.2 micron must be used to form a sintered layer having an average pore size of less than 0.1 micron. This will cause a problem in corrosion resistance of the sintered layer.
The latter inorganic porous membrane disclosed in the Japanese Non-examined Patent Publication No. 61-500221 is in the form of a tubular filter made of 99.9% alumina by weight and having an average pore size of from 2 to 20 micron, the filter being formed thereon with a filtration layer of titanum oxide having an average pore size of 0.2 micron. The publication does not disclose any porous membrane having an average pore size of less than 0.1 micron and superior in corrosion resistance.
Under the foregoing background, the problems to be solved by the present invention are summarized as follows. In general, pin holes and cracks in the thin layer of the multilayer inorganic porous membrane occur during the forming process of the thin layer. The porous thin layer is formed by coating a porous support by immersion in a sol liquid of fine particles, removal of the support from the sol liquid, drying in air, and firing. In such a process for forming the thin layer, the sol liquid permeates into pores of the support and is concentrated in the surface of the support. If the particles in the sol liquid is partly sucked into the support, pin holes will occur in the thin layer. In case the thin layer is partly thickened, cracks will occur by shrinking through drying and firing. The inventors have found the fact that the pin holes and cracks will occur if the particles in the sol liquid permeates easily into a portion of maximum pores in the porous support. It is, therefore, important to define the maximum pore size of the porous support in relation to the thin layer for preventing the forming of pin holes and cracks. In the prior art, however, the maximum pore size of the support has not been considered to prevent the forming of pin holes and cracks. The pin holes and cracks in the thin layer not only deteriorates precision of filtration but also causes the peeling off of the thin layer during washing with acid and alkali and steam sterilization.
To improve the corrosion resistance of the multilayer inorganic porous membrane, it has been considered to enhance the purity of materials. However, an impurity is inevitably contained in the material during the production process, while it is very difficult to control the sintering condition of a high purity material. In the sintering process of the high purity material, it is required to contain a sintering agent and a sintering restrainer into the material for obtaining a desired mechanical strength of the porous membrane. For these reasons, it has been found that a specific composition should be determined to obtain an inorganic porous membrane superior in corrosion resistance and capable of minimizing dissolution of components therefrom.
In a process of the production of an inorganic porous membrane having an average pore size of less than 0.1 micron, fine particles with a particle size of less than 0.2 micron or relatively large particles with a particle size of more than 0.2 micron are used as a material for the porous membrane. In a production process using the particles less than 0.2 micron, it is advantageous that spaces among the particles can be utilized to form pores in the membrane. It is, however, required to define a composition superior in corrosion resistance because of activity of the material itself. In a production process using the particles more than 0.2 micron, the material itself is relatively stable. In this process, however, spaces among the particles are shrinked by sintering to form pores in the membrane. It is, therefore, difficult to form a desired number of pores in the membrane. Moreover, such thermal shrinkage of the material will cause cracks in the membrane.